Image processing apparatus and image processing method

ABSTRACT

An image processing apparatus includes a conversion unit and a determination unit to convert image data into color material data including color material data indicating a color material amount of a chromatic color material and color material data indicating a color material amount of an achromatic color material. The conversion unit converts image data of a region of interest in an image into the color material data of the chromatic color material. The determination unit determines the color material data of the achromatic color material corresponding to the image data so that a color indicated by coloring information corresponding to the image data of the region of interest in the image approaches an achromatic color.

TECHNICAL FIELD

The present invention relates to image processing for recording an imagewith use of a clear ink.

BACKGROUND ART

Conventionally, there has been an inkjet method of forming an image on arecording medium by attaching inks, which are recording materials (colormaterials), to the recording medium, as a recording method of recording,for example, a character and an image onto a recording medium such as arecording sheet or a film.

Widely used types of ink for inkjet recording apparatuses are a dye ink,which contains dye as a color material, and a pigment ink, whichcontains pigment as a color material. The pigment ink contains, forexample, resin, water, and a color material, and has such acharacteristic that solid contents thereof such as a color material andresin are easily deposited on the surface of a recording medium,compared to the dye ink. FIG. 1 schematically illustrates pigment colormaterials deposited on a recording medium. Further, forming an image ona recording material using the pigment ink leads to a phenomenon ofcoloring of specular reflection light which is light reflected by theformed image. More specifically, when an image formed by this kind ofrecording apparatus is placed under a light source such as a spotlight,although the spotlight emits achromatic light, this light turns intocolored specular reflection light after being reflected on a recordingmedium. For example, in a color image, a region with a cyan ink laid ona large part of the region tends to look colored magenta, while amonochrome image tends to look colored yellow as a whole. Further, suchcoloring of specular reflection light tends to change in an iridescentmanner according to a change in an ink amount (discharge amount) used ina predetermined area of an image. Occurrence of coloring of specularreflection light results in deterioration of the image quality due to adifference between the color of the specular reflection light and thecolor of the diffused light.

Now, a method of measuring coloring of specular reflection light(Japanese Patent Application Laid-Open No. 2006-1.77797) will be nowbriefly described with reference to FIG. 2. A measurement sample 201 isirradiated with light by a light source 202 from a predetermined angle,and the specular reflection light reflected by the measurement sample201 is detected by a light receiver 203. The light receiver 203 detectstristimulus values XxYxZx in the International Commission onIllumination (CIE) XYZ color system. The degree how much the specularreflection light is colored is indicated as color saturation C*, whichis expressed by a*b* in the CIE L*a*b color system, based on adifference between the detected XxYxZx and tristimulus values XxYxZx ofa sample that does not cause bronzing (for example, a black polishedglass plate in which the wavelength dispersion of a reflective index issmall). Less colored specular reflection light results in reduced C*,and C* becomes zero for a sample that does not cause coloring ofspecular reflection light (in other words, C* is positioned on theorigin point on the a*b* plane).

Bronzing and thin-film interference are known as reasons that specularreflection light is colored as mentioned above.

Bronzing is a phenomenon that occurs due to the wavelength dependency ofreflection on an interface of a formed image. It is known that each inkhas a unique color to which the color of the ink is changed by abronzing phenomenon. For example, specular reflection light is coloredmagenta in a region where an image is formed by a cyan ink. JapanesePatent Application Laid-Open No. 2008-236219 discusses that, when animage is formed on a recording medium using a plurality of recordingmaterials, occurrence of bronzing is prevented by determining arecording order in which the color materials are laid, in such a mannerthat a recording material having smaller tristimulus values indicativeof bronzing is overlaid on a recording material having largertristimulus values.

However, according to the method discussed in Japanese PatentApplication Laid-Open No. 2008-236219 it is impossible to completelyoverlay a color material on another color material in an image regionusing a color material having large tristimulus values indicative ofbronzing more than a color material having small tristimulus valuesindicative of bronzing, and therefore, this method is less effective insuch a case. Especially in a highly color-saturated image region, thisineffectiveness is more remarkable due to heavy use of a singlerecording material. In other words, this conventional method leaves muchto be improved.

Another possible measure against coloring of specular reflection lightis a method of using a clear ink, which is an ink containing no colormaterial, as a recording material laid on the outermost surface of arecording medium, as illustrated in FIG. 1. A transparent clear ink hassignificantly small tristimulus values indicative of bronzing, and doesnot affect color development. Therefore, a clear ink can be used in anyimage region, and is expected to reduce coloring of specular reflectionlight more effectively.

However, this method results in a change in coloring of specularreflection light according to the discharge amount of a clear ink, sincean optical path difference occurs in reflected light between an upperlayer and a lower layer of a clear ink layer formed on a recordingmedium, and this optical path difference causes a thin-filminterference.

This coloring of specular reflection light will be now described withreference to FIG. 3. FIG. 3 schematically illustrates the result ofmeasuring the coloring of specular reflection light when a clear ink islaid on a solid surface formed by a cyan ink while changing thedischarge amount of the clear ink with use of the method discussed inJapanese Patent Application Laid-Open No. 2006-177797, and then plottingthe measured values on the a*b* plane. The numbers on the graph indicatethe discharge amount of the clear ink. This graph shows that thecoloring of specular reflection light reflected by the solid surfaceformed by the cyan ink is located in the magenta color phase, and thecoloring is rotated in the clockwise direction on the a*b* planeaccording to an increase in the amount of the clear ink. In this way,this measurement has revealed that recording a clear ink on a color inkdoes not necessarily reduce coloring of specular reflection light, andcoloring varies depending on the amount of a clear ink.

Further, coloring also varies depending on the type of a color ink laidunder a clear ink. For example, coloring caused when a predeterminedamount of a clear ink is overlaid on a solid surface formed by a cyanink is different from coloring caused when the same amount of the clearink is recorded on a solid surface formed by a magenta ink. In otherwords, just recording a predetermined amount of a clear ink on a colorink cannot completely reduce coloring in reflection on an interfacebetween the color ink and the clear ink.

SUMMARY OF INVENTION

The present invention is directed to image processing capable ofdetermining the discharge amount of a clear ink so as to comprehensivelyreduce coloring of specular reflection light.

According to an aspect of the present invention, an image processingapparatus is configured to convert image data into color material dataincluding color material data indicating a color material amount of achromatic color material and color material data indicating a colormaterial amount of an achromatic color material. The image processingapparatus includes conversion means configured to convert image data ofa region of interest in an image into the color material data of thechromatic color material, and determination means configured todetermine the color material data of the achromatic color materialcorresponding to the image data so that a color indicated by coloringinformation corresponding to the image data of the region of interest inthe image approaches an achromatic color.

According to another aspect of the present invention, an imageprocessing method, for converting image data into color material dataincluding color material data indicating a color material amount of achromatic color material and color material data indicating a colormaterial amount of an achromatic color material, includes convertingimage data of a region of interest in an image into the color materialdata of the chromatic color material, and determining the color materialdata of the achromatic color material corresponding to the image data sothat a color indicated by coloring information corresponding to theimage data of the region of interest in the image approaches anachromatic color.

Further features and aspects of the present invention will becomeapparent from the following detailed description of exemplaryembodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate exemplary embodiments, features,and aspects of the invention and, together with the description, serveto explain the principles of the invention.

FIG. 1 schematically illustrates color materials deposited on arecording medium.

FIG. 2 illustrates a method of quantifying coloring of specularreflection light.

FIG. 3 schematically illustrates coloring of specular reflection lightwhich is plotted on the a*b* plane.

FIG. 4A illustrates a principle of a first exemplary embodiment of thepresent invention.

FIG. 4B illustrates the principle of the first exemplary embodiment ofthe present invention.

FIG. 5 is a block diagram illustrating a configuration of a printingsystem according to the first exemplary embodiment.

FIG. 6 illustrates a specular reflection light coloring table accordingto the first exemplary embodiment.

FIG. 7 is a flowchart illustrating processing of generating the specularreflection light coloring table.

FIG. 8 is a block diagram, illustrating a functional configuration ofclear ink amount determination processing according to the firstexemplary embodiment.

FIG. 9 is a flowchart illustrating the clear ink amount determinationprocessing according to the first exemplary embodiment.

FIG. 10 illustrates multipass recording of color inks.

FIG. 11 illustrates multipass recording of a clear ink.

FIG. 12 is a block diagram illustrating a functional configuration ofclear ink amount determination processing according to a secondexemplary embodiment of the present invention.

FIG. 13 is a flowchart illustrating the clear ink amount determinationprocessing according to the second exemplary embodiment.

FIG. 14 is a block diagram illustrating a functional configuration ofclear ink amount determination processing according to a third exemplaryembodiment of the present invention.

FIG. 15 is a flowchart illustrating the clear ink amount determinationprocessing according to the third exemplary embodiment.

FIG. 16 is a block diagram illustrating a functional configuration ofclear ink amount determination processing according to a fourthexemplary embodiment of the present invention.

FIG. 17A illustrates a resolution conversion according to the fourthexemplary embodiment.

FIG. 17B illustrate a resolution conversion according to the fourthexemplary embodiment.

FIG. 18 is a graph illustrating a principle of a fifth exemplaryembodiment of the present invention.

FIG. 19 schematically illustrates the principle of the fifth exemplaryembodiment.

FIG. 20 is a block diagram illustrating a functional configuration ofclear ink amount determination processing according to the fifthexemplary embodiment.

FIG. 21 illustrates a specular reflection light coloring table accordingto the fifth exemplary embodiment.

FIG. 22 is a flowchart illustrating the clear ink amount determinationprocessing according to the fifth exemplary embodiment.

FIG. 23 is a graph illustrating the relationship between the clear inkamount and coloring according to the fifth exemplary embodiment.

FIG. 24 illustrates a specular reflection light coloring table accordingto the fifth exemplary embodiment.

FIG. 25 is a block diagram illustrating a functional configuration ofclear ink amount determination processing according to a sixth exemplaryembodiment of the present invention.

FIG. 26 is a block diagram illustrating a clear ink amount determinationprocessing unit according to the sixth exemplary embodiment.

DESCRIPTION OF EMBODIMENTS

Various exemplary embodiments, features, and aspects of the inventionwill be described in detail below with reference to the drawings.

In exemplary embodiments of the present invention, inks, which are usedas recording materials, are expressed by the name of the color thereofsuch as cyan, magenta, yellow, black, clear (colorless or almostcolorless ink), red, green, and blue. Colors, data of the colors, or thecolor phases of the colors are denoted by the initial thereof such as C,M, Y, K, CL, R, G, and B. More specifically, “C” represents the cyancolor, data of the cyan color, or the color phase of the cyan color.Similarly, “M” represents magenta, “Y” represents yellow, “K” representsblack, “R” represents red, “G” represents green, and “B” representsblue. “CL” represents colorless (transparent) color, or data thereof.Further, “coloring of specular reflection light” may be referred to asjust “coloring” or “color”. A value indicating coloring, such as an a*b*value in the CIE-L*a*b* color system, may be referred to as “coloringinformation”.

As will be used herein, “area” is a smallest unit to which ON/OFF of adot is defined. In connection of this definition, “image data” in colormatching, color separation, and gamma correction, which will bedescribed below, refers to a set of pixel data which is a processingtarget. Each pixel data indicates an 8-bit graduation value.

Further, “pixel data” in halftoning refers to pixel data which is aprocessing target itself. Halftoning converts pixel data containing an8-bit gradation value as mentioned above into pixel data (index data)containing a 4-bit gradation value. In the following description, theterm “pixel” is used to refer to the smallest constituent unit for whichthe discharge amount of a clear ink can be changed, unless otherwiseindicated.

A first exemplary embodiment of the present invention employs a methoddeveloped by applying the error diffusion method used in halftoning as amethod of determining a clear ink amount. Generally, the error diffusionmethod processes a pixel value as an error, while the first exemplaryembodiment processes coloring of specular reflection light as an error.

Further, an image forming apparatus according to the first exemplaryembodiment uses a clear ink, and forms such an image that the clear inkis overlaid on a color ink which is a chromatic color material. The term“overlay” means recording an image in such a manner that a certain inkis recorded on a recording medium as the last ink in a recording order,as illustrated in FIG. 1. The overlaying method may be any methodcapable of resulting in overlaying of a clear ink, although the presentexemplary embodiment will be described based on an example using themethod discussed in Japanese Patent Application Laid-Open No.2008-162094.

First, the principle of the first exemplary embodiment will bedescribed. For convenience of description, the description will begiven, with reference to a one-dimensional image having only one pixelin the longitudinal direction. However, the basic principle is alsoapplicable to a two-dimensional image which is a commonly processedimage. FIG. 4A schematically illustrates color materials deposited on arecording medium 201 when an image is formed using only color inks 402without using a clear ink. On the other hand, FIG. 4B schematicallyillustrates image formation with a clear ink amount overlaid on eachpixel. A different color ink 402 is deposited on each pixel on therecording medium 201, and coloring of specular reflection light occursin a different manner on each pixel. A clear ink amount, which is colormaterial data, is determined, starting from the leftmost pixel. First,as a clear ink amount for the leftmost pixel, a clear ink amount 403 isdetermined so as to maximally reduce coloring at that pixel based oncoloring information acquired in advance. The coloring informationindicates coloring when an overlaid clear ink amount is changed, withrespect to each pixel value. The details of the coloring informationwill be described below.

When the coloring cannot be completely eliminated, i.e., the light doesnot become achromatic, a difference occurs between the coloring thatoccurs at the leftmost pixel, and achromatic coloring. This differenceis diffused to the next pixel as an error in the error diffusion method.For example, if specular reflection light is colored green at a pixelwith a predetermined amount of a clear ink overlaid thereon, redcoloring is diffused to the next pixel as an error.

Next, a clear ink amount for the next pixel is determined so that actualcoloring approaches the coloring diffused as an error. In theabove-described example, a clear ink amount is determined so that thecoloring approaches red at that pixel based on the coloring informationacquired in advance. As in the above-described example, since green andred are complementary colors to each other, the global coloring looksachromatic when it is observed from a big-picture perspective, eventhough coloring locally occurs at each pixel when it is observed pixelby pixel. In other words, it is possible to reduce global coloring bydetermining a clear ink amount to be overlaid for each pixel in such amanner that the coloring phenomena at the respective pixels cancel outeach other. Coloring of specular reflection light is expressed in twodifferent manners, i.e., local coloring and global coloring, becausecoloring of specular reflection light depends on the observation scale.As will be used herein, “global coloring of specular reflection light”refers to coloring averaged in a scope wider than the scope that humanscan resolve coloring of specular reflection light. On the other hand,“local coloring of specular reflection light” refers to coloring inseveral 10 micrometer order size in a scope that humans cannot resolvecoloring of specular reflection light. In other words, a change incoloring minute more than the resolution of a human's eye is sensed asaveraged coloring.

Further, when the coloring cannot be completely eliminated by thecancelling-out effect, a difference occurs between the coloring thatoccurs at the second pixel from left, and achromatic coloring.Sequential execution of the processing of diffusing such a difference tothe next pixel as an error enables a reduction in the global coloring ofspecular reflection light.

Next, the processing according to the first exemplary embodiment will bedescribed in detail. FIG. 5 is a block diagram illustrating internalconfigurations of a host apparatus (personal computer (PC)) and an inkjet recording apparatus (inkjet printer) constituting a printing systemaccording to the first exemplary embodiment. The inkjet printer isprovided with a color ink containing pigment as a color material foreach of four colors, cyan (C), magenta (M), and yellow (Y), which arebasic colors, and black (K). Further, the inkjet printer is providedwith a clear ink (CL) which is a colorless (transparent) ink, inaddition to these four colors. The inkjet printer prints data by usingthe inks of these five colors in total. Therefore, the inkjet printerincludes a recording head 511 for discharging the inks of these fivecolors.

An application and a printer driver are provided as programs whichoperate under an operating system on the PC. The application 501performs the processing for generating image data to be printed by theprinter. This image data, or previous data, for example, before beingedited can be introduced into the PC via various kinds of media. The PCcan acquire image data captured by a digital camera, which is, forexample, in the Joint Photographic Experts Group (JPEG) format, via acompact flash (CF) card. Further, the PC can also acquire image datascanned by a scanner, which is, for example, in the Tag Image FileFormat (TIFF) format, and image data stored in a compact disc read onlymemory (CD-ROM). Further, the PC can also acquire image data on a website via the Internet. These acquired pieces of image data are displayedon a monitor of the PC, and for example, is edited and processed via theapplication 501. After that, the image data is converted into, forexample, Red Green Blue (RGB) image data expressed by R, G, and Bsignals under Standard Red Green Blue (sRGB). Then, this RGB image datais supplied to the printer driver according to a print instruction.

The printer driver performs various kinds of processing, i.e., colormatching 502, color separation 503, clear ink amount determination 504,gamma correction 505, halftoning 506, and print data generation 507.

The color matching 502 performs gamut mapping. The color matching 502uses a three-dimensional look-up table (LUT) to map the gamut reproducedby R, G, and B signals under the sRGB standard into the gamut reproducedby the inkjet printer. Then, the color matching 502 uses this LUT andinterpolation calculation to perform a data conversion for converting8-bit RGB data to RGB data in the gamut of the printer.

The color separation 503 converts RGB data into color separation data(CMYK data) corresponding to the combination of inks that can reproducethe color indicated by the RGB data, based on the RGB data after thegamut mapping. This processing is performed by using interpolationcalculation in addition to a three-dimensional LUT, similarly to thecolor matching. The output thereof is 8-bit data for each color, anduses a value corresponding to a color material amount of each colormaterial of C, M, Y, and K.

The clear ink amount determination processing 504 determines a clear inkamount to be overlaid on each pixel by referring to a specularreflection light coloring table 512. The details thereof will bedescribed below.

The gamma correction 505 applies a gradation value conversion to data ofeach color in the color separation data acquired by the color separation503 and the clear ink amount determination processing 504. Morespecifically, the gamma correction 505 performs such a conversion thatthe color separation data linearly corresponds to the gradationcharacteristic of the inkjet printer by using a one-dimensional LUTaccording to the gradation characteristic of each color ink of the inkjet printer. The clear ink is transparent, and therefore the gammacorrection is not applied to the color material amount of the clear ink.

The halftoning 506 performs quantization for converting each signal ofthe C, M, Y, K, and CL signals of the 8-bit color separation data(CMYKCL data) into 4-bit image data. In the present exemplaryembodiment, 8-bit data is converted into and output as 4-bit data withuse of the error diffusion method. This 4-bit image data is index dataindicating a layout pattern in dot layout pattern assignment processingin the ink jet printer. The quantization is not limited to the errordiffusion method, and may be performed by for example, the thresholdvalue processing with use of a dither matrix. Alternatively, thequantization may be performed by establishing a relationship among therespective C, M, Y, K, and CL signals.

Finally, the print data generation 507 generates print data by addingprint control information to print data containing the 4-bit index data.

The above-described kinds of processing of the application 501 and theprinter driver are executed by a central processing unit (CPU) accordingto the programs thereof. At this time, the CPU reads the programs from aread only memory (ROM) or a hard disk to use them. The CPU uses a randomaccess memory (RAM) as a work area when executing the processing.

The inkjet printer includes a dot layout pattern assignment unit 508, amask data conversion unit 509, a head drive circuit 510, and a recordinghead 511.

The dot layout pattern assignment unit 508 determines a dot layout foreach pixel corresponding to an actual print image, according to a dotlayout pattern corresponding to 4-bit index data (gradation valueinformation) which is print image data. The above-described halftoning506 reduces the level number of multivalued density information of 256values (8-bit data) to gradation value information of 9 values (4-bitdata). However, information that the inkjet printer can actually recordis only two values information indicating whether to record an ink ornot. The dot layout pattern assignment unit 508 assigns a dot layoutpattern corresponding to a gradation value (level 0 to 8) of a pixel toeach pixel expressed by 4-bit data indicating level 0 to 8, which is anoutput value from the halftoning 506. This assignment defines ON/OFF ofa dot for each of a plurality of areas in one pixel. In other words,whether a dot is generated is defined for each of the plurality of areasin one pixel, and two-value discharge data, i.e., “1” or “0” is set toeach area in one pixel.

The mask data conversion unit 509 applies mask processing to 1-bitdischarge data acquired from the dot layout pattern assignment. In otherwords, the mask conversion unit 509 generates discharge data of eachscanning so that the recording head 511 can complete recording on ascanning region having a predetermined width by a plurality of scanningprocesses. At this time, the mask data conversion unit 509 applies maskprocessing so that the clear ink is discharged during the last scanningof the plurality of scanning processes. In other words, the mask dataconversion unit 509 generates discharge data causing the clear ink to belaid on the outermost surface on a paper relative to the other inks. Thedetails of the mask processing will be described below.

Discharge data C, M, Y, K, and CL for each scanning is transmitted tothe head drive circuit 510 at appropriate timing, whereby the recordinghead 511 is driven to discharge each ink according to the dischargedata.

The above-described dot pattern assignment unit 508 and the mask dataconversion unit 509 in the inkjet printer are executed under control ofa CPU, which constitutes a not-shown control unit, with use of ahardware circuit dedicated to each of them. These kinds of processingmay be performed by the CPU according to programs, or may be performedby, for example, a printer driver in the PC.

The halftoning 506 and the print data generation 507 have been describedassuming that they are performed by the printer driver installed in thePC, but the present exemplary embodiment is not limited thereto. Thesystem may be configured in such a manner that the halftoning isperformed in the printer.

Next, each processing according to the present exemplary embodiment willbe described in further detail. The present exemplary embodimentdetermines a clear ink amount by referring to the specular reflectionlight coloring table and using the method developed by applying theerror diffusion method. The specular reflection light coloring table isa table indicating the relationship between the clear ink amount andcoloring of specular reflection light. First, the specular reflectionlight coloring table will be described, and then the clear ink amountdetermination processing will be described.

The specular reflection light coloring table stores required data sothat coloring of specular reflection light when the clear ink amount ischanged for each ROB value can be acquired as an a*b* value in theCIE-L*a*b color system. FIG. 6 illustrates specific contents of thestored data. Referring to FIG. 6, the first to third columns contain therespective RGB values, the fourth column contain the ink value of theclear ink, and the fifth and sixth columns contain the values of a* andb*, as which coloring of specular reflection light is expressed in theL*a*b* color system.

Specular reflection light coloring data expressed by the values of a*and b* can be acquired by printing a patch image reproduced by acombination of an RGB value and an ink value of the clear ink on arecording medium, and measuring the coloring by a measurement apparatus.Although FIG. 6 illustrates an example of a table containing specularreflection light coloring data corresponding to 8 gradation values eachincremented by 32 levels as the ink values of the clear ink, the numberof bits of the ink values of the clear ink is not limited thereto. Whenthe RGB values are gradation values of 256 levels for each color,specular reflection light coloring data is required for each ofcombinations (256×256×256×8=134217728). These combinations are enormousas a total, and therefore take a lot of measurement time and occupy asignificant data amount.

Therefore, the present exemplary embodiment utilizes tetrahedroninterpolation with use of eight pieces of specular reflection lightcoloring data, and obtains the other pieces of specular reflection lightcoloring data by an interpolation calculation. The interpolation methodmay be, for example, cubic interpolation. Specular reflection lightcoloring data is not limited to data corresponding gradation values at asame interval for each RGB value, and may be measured at variableintervals.

Further, the specular reflection light coloring table is not limited tothe above-described structure, and may have any structure that canrelate combinations constituted by the signal values of the input colorsignals and the ink values of the clear ink to coloring of specularreflection light. Further, the input signals may be CMYK values, and maybe switched between the RGB values and the CMYK values automatically oraccording to a mode specified by a user.

FIG. 7 is a flowchart illustrating processing of generating the specularreflection light coloring table. In step S701, the CPU sequentially setsthe RGB values corresponding to the first to third columns illustratedin FIG. 6. In step S702, the CPU acquires the ink value of the color inkrequired to form the image indicated by the RGB value set in step S701.More specifically, the CPU acquires the CMYK data, which can be obtainedby the color separation 503 illustrated in FIG. 5, from the set RGBvalue. In step S703, the CPU sets the ink value of the clear inkcorresponding to the fourth column illustrated in FIG. 6 from a smallvalue to a large value in ascending order. In step S704, the CPUdetermines whether a total amount of the color ink amount indicated bythe ink value of the color ink, and the clear ink amount indicated bythe ink value of the clear ink exceeds a predetermined upper limit valuefor the recording medium. The upper limit value is set because there isa limitation to an ink amount that a receiver layer of a recordingmedium can hold. In case that the total ink amount is more than theupper limit value (NO in step S704), the CPU sets the next RGB value. Incase that the total ink amount is equal to or less than the upper limitvalue (YES in step S704), the processing proceeds to step S705, in whichthe CPU generates patch image data with use of the ink values acquiredby the above-described steps, and causes the inkjet printer to print thegenerated patch image data. In step S706, the CPU inputs a measurementvalue acquired by measuring the coloring of specular reflection lightreflected on the printed patch image. In step S707, the CPU determineswhether the CPU sets all of the ink values of the clear inkcorresponding to the fourth column illustrated in FIG. 6. In case thatthere is any ink value left to be set (NO in step S707), the CPU repeatsstep S703 and the steps thereafter with a changed (increased) clear inkamount. In case that the CPU sets all of the ink values (YES in stepS707), the processing proceeds to step S708. Execution of these stepsenables acquisition of specular reflection light coloring data when theink value of the clear ink is changed for a certain RGB value. Next, instep S708, the CPU determines whether the CPU sets all of the RGBvalues, which are set in the step S701. In case that there is any RGBvalue left to be set (NO in step S708), the CPU repeats step S701 andthe steps thereafter after setting the next RGB value. In case that theCPU sets all of the RGB values that should be measured (YES in stepS708), the CPU stores the acquired specular reflection light coloringdata in the specular reflection light coloring table in such a mannerthat the data is related to the respective RGB values. Then, theprocessing is ended.

Execution of the above-described steps enables acquisition of thespecular reflection light coloring table when the ink value of the clearink is changed for all of the RGB values.

Next, the clear ink amount determination processing will be described.The processing is executed by controlling local coloring of specularreflection light on an image, and causing global coloring of specularreflection light within a range enabling an additive color mixture oflocal coloring of specular reflection light.

FIG. 8 is a block diagram illustrating a functional configuration of theclear ink amount determination processing 504. An input unit 801 selectspixel data of a pixel of interest to be processed one by one from imagedata constituted by an array of a plurality of pixels, and inputs inputpixel data. The specular reflection light coloring table 802 retains theabove-described the specular reflection light coloring table. A targetdata input unit 803 inputs target data. A candidate color acquisitionunit 804 acquires candidate colors for coloring of specular reflectionlight that can be realized at a pixel of interest. A cumulative errormemory 805 stores a cumulative error. An addition unit 806 adds thecumulative error stored in the cumulative error memory 805 to the targetdata. A determination unit 807 determines coloring to be realized at thepixel of interest from the candidate colors acquired by the candidatecolor acquisition unit 804, and identifies a clear ink amount requiredfor the determined coloring. An error diffusion unit 808 diffuses acoloring error to a pixel in the vicinity of a pixel of interest. Anoutput unit 809 outputs, to the gamma correction processing 505, thedetermined clear ink amount as output data one by one for each pixel, orcollectively for all pixels.

FIG. 9 is a flowchart illustrating an operation of the clear ink amountdetermination processing 504 illustrated in FIG. 8. The clear inkdetermination processing is first started with an uppermost and leftmostpixel on an image as a pixel of interest, and then is continued whileswitching the pixel of interest to the right pixel one by one. After theprocessing of an uppermost and rightmost pixel, the pixel of interest isthen shifted to a leftmost pixel on a next pixel row below. Theprocessing is continued in this order, and is ended with the processingof a lowermost and rightmost pixel on the image.

Upon a start of the processing, in step S901, the input unit 801 inputspixel data of a pixel of interest.

In step 902, the candidate color acquisition unit 804 refers to thespecular reflection light coloring table 802 for the data correspondingto the input pixel data, acquires eight pieces of data (ai*, bi*) (i=1to 8) of candidate colors for the coloring of specular reflection light,and outputs them to the determination unit 807. The eight candidatecolors have different clear ink amounts with respect to a same RGBvalue.

Then, in step S903, one piece of target data (at*, bt*) is input. In thepresent exemplary embodiment, the values of the target data are set as(at*=0, bt*=0) regardless of the pixel, so as to reduce global coloringof specular reflection light.

Next, in step S904, the addition unit 806 adds, to the target data, onecumulative error (as*, bs*) corresponding to the pixel position of thepixel of interest, which is stored in the cumulative error memory 805,by using the following equation.

at*←at*+as*,bt*←bt*+bs*  [Math.1]

The arrows in the above equation represent substitution. Assuming that xis the horizontal pixel position of the pixel data of a pixel ofinterest, the cumulative error memory 805 includes one storage area(Sa0, Sb0) and w storage areas (Sa(x), Sb(x)) (x=integer of 1 to W).Each storage area stores an error (as*, bs*) to be applied to a pixel ofinterest. The value of a cumulative error is acquired by a method thatwill be described below. At the onset of the processing, all of thestorage areas are initialized to an initial value (Sa(x)=0, Sb(x)=0).

Next, the determination unit 807 calculates color differences betweenthe a*b* values of the eight candidate colors, and the target data withthe cumulative error added thereto. The color difference is calculatedby the following equation.

√{square root over (((at*−ai*)²+(bt*−bi*)²))}{square root over(((at*−ai*)²+(bt*−bi*)²))}(i=1 to 8)  [Math.2]

In step 905, the determination unit 807 identifies the candidate color(ai*. In*) that has the smallest color difference by this equation,determines the clear ink amount corresponding to the identifiedcandidate color, and outputs the determined clear ink amount to theoutput unit 809. The determination unit 807 outputs Sa=at*−ai* andSb=bt*−bi*, which is an error of this candidate color, to the errordiffusion unit 808. For example, when coloring that occurs at a pixel ofinterest is green, the error thereto (coloring to be diffused) is red.The candidate color is not limited to a color having a smallest colordifference, and may be a color having a small color difference. Further,in the above description, an error is calculated by subtracting coloringof a candidate color from target data. However, the error calculation isnot limited to this method, and may be performed by subtracting targetdata from a candidate color.

Then, the error diffusion unit 808 performs the following errordiffusion processing according to the horizontal position of the pixelof interest in the image. That is, the error diffusion unit 808calculates an error to be stored in the storage area S0 and S(x)according to the following equation, and stores it in the cumulativeerror memory 805.

(Sa(x+1),Sb(x+1))←(Sa(x+1)+Sa× 7/16,Sb(x+1)+Sb× 7/16)(x<W)

(Sa(x−1),Sb(x−1))←(Sa(x−1)+Sa× 3/16,Sb(x−1)+Sb× 3/16)(x>1)

Sa(x),Sb(x)←(Sa0+Sa× 5/16,Sb0+Sb× 5/16)(1<x<W)

(Sa(x),Sb(x))←(Sa0+Sa× 8/16,Sb0+Sb× 8/16)(x=1)

(Sa(x),Sb(x))←(Sa0+Sa× 13/16,Sb0+Sb× 13/16)(x=W)

(Sa0(x),Sb0(x)←(Sa× 1/16,Sb× 1/16)(x<W)

(Sa0(x),Sb0(x))←(0,0)(x=W)  [Math.3]

In this way, in step S906, the error diffusion processing for one pixelis completed.

Lastly, in step S907, it is determined whether steps S901 to S906 areapplied to all of the pixels in the image. In case that there is anypixel that the steps are not yet applied (NO in step S907), theprocessing returns to step S901, while in case that the steps areapplied to all of the pixels (YES in step S907), the clear ink amountdetermination processing is ended.

The mask data conversion unit 509 converts discharge data for one bitwhich is generated by the dot layout pattern assignment unit 508 intodischarge data for each scanning.

Whether dot is ON or OFF in each area on a recording medium is alreadydetermined by the processing of the dot layout pattern assignment unit508, and, therefore, a desired image can be recorded on the recordingmedium by inputting the generated binary discharge data even without anyconversion into the drive circuit of the recording head 511. However,the ink jet recording apparatus employs the multipass recording methodto reduce deterioration of an image quality that may be caused due to,for example, a variation among ink droplet discharge characteristics ofindividual nozzles and a variation in the accuracy of conveying arecording medium. Therefore, the multipass recording in the presentexemplary embodiment will be described below.

A method of recording the color inks by the multipass method will bedescribed with reference to FIG. 10. FIG. 10 schematically illustrates arecording head and a recording pattern. The recording head 1001 isdivided into five nozzle groups, the first nozzle group to the fifthnozzle group. Each nozzle group includes four nozzles. In a mask pattern1002, the black blocks indicate an area to be recorded by each nozzle.In color ink recording, the mask pattern corresponding to the fifthnozzle group is white as a whole, which means that the fifth nozzlegroup records nothing. The recording head 1001 is configured in such amanner that the patterns recorded by the respective nozzle groups are ina complementary relationship to one another, and overlaying all of thepatterns of the first to fourth nozzle groups results in a completion ofrecording on a region corresponding to 4×4 areas.

The respective patterns 1003 to 1006 illustrate the process during whichan image is being completed by repeating a scanning operation. Each timescanning of each nozzle group is completed, the recording medium isconveyed by a distance corresponding to the width of the nozzle group inthe direction indicated by the arrow in FIG. 10. Therefore, execution offour times of scanning finally completes recording of an image withcolor inks on a predetermined recording region (a region correspondingto the width of each nozzle group) of the recording medium. In thepresent exemplary embodiment, the term “the number of passes” is used torefer to the number of times of scanning required to complete an imageon the predetermined recording region. In this way, forming apredetermined recording region by a plurality of times of scanningperformed by a plurality of nozzle groups is beneficial in reducingdeterioration of an image quality which might be caused due to, forexample, a variation among individual nozzles and a variation in theaccuracy of conveying a recording medium.

A method of recording the clear ink by the multipass method will bedescribed with reference to FIG. 11. FIG. 11 schematically illustrates arecording head and a recording pattern. The clear ink is recorded byscanning of the fifth nozzle group, which is not used in the color inkrecording, according to a mask pattern 1101. The present exemplaryembodiment is described based on an example of recording the clear inkby the last one time of scanning. However, the mask pattern forrecording the clear ink is not limited thereto, and may be any maskpattern capable of recoding the clear ink on the outermost surface of arecording medium. For example, the inkjet printer may further include asixth nozzle group, and use a mask pattern for recording the clear inkby a plurality of times of scanning.

The mask data conversion unit 509 generates mask data for each ink byperforming a logical AND (logical multiplication) between 1-bitdischarge data generated by the dot layout pattern assignment unit 508,and the mask pattern 1002 illustrated in FIG. 10 and the mask pattern1101 illustrated in FIG. 11. Generation of mask data in this way leadsto such recording that the clear ink is laid on the uppermost layer on arecording medium, thereby enabling overlaying of the clear ink.

As described above, according to the present exemplary embodiment, it ispossible to reduce coloring of specular reflection light by determininga clear ink amount to be overlaid on each pixel so as to reduce globalcoloring of specular reflection light on an image.

As the first exemplary embodiment, the method of reducing globalcoloring of specular reflection light has been described. However, themethod according to the first exemplary embodiment may cause coloringrecognizable when an image is studied from up close. This is because theclear ink amount is determined so that coloring in a whole image becomescomprehensively achromatic. In other words, an error (coloring) thatoccurs at a certain pixel is subsequently cumulated, and the cumulatederror is released in a region away from the region where the pixelexits.

Therefore, as a second exemplary embodiment of the present invention, adescription will be given of a method in which an upper limit is set toaccumulation of an error so as to prevent an error that occurs at acertain pixel from being released in a distant region. The secondexemplary embodiment will be briefly described, mainly focusing ondifferences from the first exemplary embodiment.

FIG. 12 is a block diagram illustrating a functional configuration ofthe clear ink amount determination processing 504 according to thesecond exemplary embodiment. The second exemplary embodiment includes acumulative error correction unit 1201 between the cumulative errormemory 805 and the addition unit 806 illustrated in FIG. 8. Thecumulative error correction unit 1201 corrects an error calculated bythe cumulative error memory 805 based on a predetermined limit value setin advance as an upper limit of an error. The limit value is set as, forexample, amax*=100, bmax*=100. However, the limit value is not limitedto 100, and may be determined in consideration of the degree ofaccumulation of an error.

FIG. 13 is a flowchart illustrating clear ink amount determinationprocessing according to the second exemplary embodiment. The steps otherthan cumulative error correction are the same as the steps in theflowchart of FIG. 9 according to the first exemplary embodiment, and,therefore, the descriptions thereof will be omitted. In step S1301, incase that the a* value, which is an error input from the diffusionprocessing in step S906, is equal to or smaller than amax* (a*< or=amax*), the a* value is not corrected. On the other hand, in case thatthe a* value is larger than amax* (a*>amax*), the a* value is correctedto zero. The same correction is also applied to the b* value.

In step S907, it is determined whether steps S901 to S906 are performedon all of the pixels in the image. In case that there is any pixel leftto be processed (NO in step S907), the processing returns to step S901.On the other hand, in case that all of the pixels are processed (YES instep S907), the clear ink amount determination processing is ended.

According to the above-described processing, the data stored in thecumulative error memory 805 is initialized to zero, in case that itexceeds the upper limit. As a result, it is possible to prevent an errorthat occurs at a certain pixel from being released in a distant region.

The second exemplary embodiment has been described based on an examplein which the data stored in the cumulative error memory 805 exceeds theupper limit. However, another method may be used to prevent an errorfrom being released in a distant region. For example, an upper limit isdetermined to the number of times of cumulating errors into thecumulative error memory 805, and the number of times of accumulation iscounted. When the counted number of times of accumulation exceeds theupper limit, the a* value and the b* value are corrected to zero. Therange of a position to which an error is diffused may be limited in thisway.

The method aiming at a global reduction in coloring that occurs at eachpixel has been described as the first and second exemplary embodiments.As a third exemplary embodiment of the present invention, a descriptionwill be given of a method of determining a clear ink amount so as tomake coloring most inconspicuous at each pixel, i.e., make coloring mostachromatic.

As mentioned above, coloring that occurs when a clear ink is overlaid onan color ink varies depending on an overlaid amount of the clear ink. Inother words, a certain clear ink amount generates a condition minimizingthe degree of coloring. Therefore, in case that coloring at each pixelis made inconspicuous, global coloring can be also reduced. The thirdexemplary embodiment will be briefly described, mainly focusing ondifferences from the above-described exemplary embodiments.

FIG. 14 is a block diagram illustrating a functional configuration ofthe clear ink amount determination processing 504 according to the thirdexemplary embodiment. A determination unit 1401 receives candidatecolors acquired by the candidate color acquisition unit 804, and targetdata input by the target data input unit 803. The determination unit1401 determines a clear ink amount at a pixel of interest, and outputsit to the output unit 809.

FIG. 15 is a flowchart illustrating the clear ink amount determinationprocessing 504 according to the third exemplary embodiment. In stepS1501, the determination unit 1401 calculates color differences betweenthe a*b* values of the eight candidate colors, and the target data, andidentifies the candidate color that has the smallest color difference.The determination unit 1401 refers to the specular reflection lightcoloring table 802, and outputs the clear ink amount corresponding tothe identified candidate color to the output unit 809.

A clear ink amount is determined for each pixel by performing theabove-described processing on all of the pixels.

The third exemplary embodiment has been described based on an example inwhich color differences are calculated between the a*b* values of eightcandidate colors, and target data, but the third exemplary embodiment isnot limited to this example. For example, the determination unit 1401may be omitted. In this case, in the specular reflection light coloringtable 802, relationships are established in advance between the inputsignal values illustrated in FIG. 6 and clear ink values capable ofminimizing coloring to the input signal values. Then, the candidatecolor acquisition unit 894 refers to this specular reflection lightcoloring table 802 to output the clear ink value corresponding to aninput signal value input by the input unit 801 to the output unit 809,whereby the same effect can be provided.

Execution of the above-described processing enables a determination of aclear ink amount so as to minimize coloring at each pixel. As a result,global coloring of specular reflection light can be also reduced.Further, the third exemplary embodiment does not diffuse an error(coloring), and therefore realizes simple and fast processing, comparedto the first and second exemplary embodiments.

This processing has been described as the method of determining a clearink amount capable of minimizing coloring at each pixel. According tothis method, a clear ink amount is fixedly determined according to anRGB value on which the clear ink is overlaid. However, fixedlydetermining a clear ink amount at an image region including adjacentpixels having a same RGB value or an image region with a clear inkamount of zero may result in occurrence of uneven glossiness betweenthat image region and an image region without a uniform clear amount.

This problem can be solved by overlaying different clear ink amounts atan image region having a same RGB value. Possible methods thereforinclude, for example, a method of setting a variation in target data, ora method of changing the processing by the determination unit 1401

The method of setting a variation in target data is realized by a methodof providing a random number generation unit which is not illustrated inFIG. 14, and adding a value to target data according to a generatedrandom number, or a method of performing and storing different targetdata for each pixel, and referring to it for each pixel.

On the other hand, as the method of changing the processing by thedetermination unit 1401, the processing can be changed so that thedetermination unit 1401 identifies a candidate color having the secondor third least color difference, instead of identifying a candidatecolor having the least color difference, and outputs the correspondingdata. In this case, the ranking of a color difference to select acandidate color for each pixel may be determined by providing a randomnumber generator not illustrated in FIG. 14 to determine the rankingaccording to a generated random number, or storing data of predeterminedranking to determine the ranking by referring to the stored data atappropriate timing when the determination unit 1401 makes a comparison.

Execution of the above-described processing enables a reduction inoccurrence of uneven glossiness while reducing coloring at each pixel,even if there is an image region including adjacent pixels having a sameRGB value, or an image region having a clear ink amount of zero.

The first to third exemplary embodiments set a unit having the same sizeas a pixel on an input image as a smallest unit of a region for which aclear ink amount is determined in the clear ink amount determinationprocessing. However, a problem arises in that an input image having alarge number of pixels increases a time required for the processing.

Generally, in an ordinary image, adjacent pixels often have similar ROBvalues, although each pixel has a different RGB value. Therefore, as afourth exemplary embodiment of the present invention, a description willbe given of a method of setting adjacent pixels as a smallest unit of aregion for which a clear ink amount is determined, assuming thatadjusting pixels are similar, and determining a clear ink value for eachof that smallest unit. The fourth exemplary embodiment will be brieflydescribed, mainly focusing on differences from the above-describedexemplary embodiments.

FIG. 16 is a block diagram illustrating a functional configuration ofthe clear ink amount determination processing 504 according to thefourth exemplary embodiment. The fourth exemplary embodiment includes aresolution con version unit 1601 as pre-processing for the clear inkamount determination processing 504, and a resolution conversion unit1602 as post-processing for the clear ink amount determinationprocessing 504.

The resolution conversion unit 1601 applies a low resolution conversionto an image after the color matching, and outputs an image having alower resolution than the resolution of the input data image. As oneexample, it is assumed that the resolution of an image after the colormatching is 600 dpi, and an image having a resolution of 300 dpi isoutput as the image after the resolution conversion. The resolutionconversion method may be embodied by a known method. For example, thepixel value of a pixel after the resolution conversion is an average ofpixel values of four pixels surrounding the pixel before the resolutionconversion. FIG. 17A schematically illustrates the processing applied bythe resolution conversion unit 1601 to a pixel of interest and fourpixels surrounding the pixel of interest.

The resolution conversion unit 1602 applies a high resolution conversionto an image after the clear ink amount determination processing, andreturns the resolution of the image to the resolution of the image afterthe color matching. As one example, it is assumed that the resolution ofan image after the color matching is 600 dpi, the resolution of theimage after the processing by the resolution conversion unit 1601 is 300dpi, and an image having a resolution of 600 dpi is output as the imageafter the resolution conversion. The resolution conversion method may beembodied by a known method. In other words, the pixel value of a pixelafter the processing by the resolution conversion unit 1602 is returnedto the pixel value before the processing by the resolution conversionunit 1601, while this pixel has the same clear ink amount as that of thesurrounding pixels (300 dpi) before the processing by the resolutionconversion unit 1602. The conversion may be applied to clear ink amountsof surrounding pixels, instead of surrounding pixels. FIG. 17Bschematically illustrates the processing that the resolution conversionunit 1602 applies to a pixel of interest and four pixels surrounding thepixel of interest. It should be noted that the above-describedresolution conversion is applied only to the clear ink amountdetermination, and is not applied to the determination of ink amounts ofthe color inks.

In the above description, the resolution conversion unit 1601 reduces aresolution by half, but may reduce a resolution to a quarter or anone-eighth of the original resolution.

Execution of the above-description processing enables a reduction in acalculation time required for the clear ink amount determinationprocessing although requiring an additional time for the processing bythe resolution conversion units 1601 and 1602, compared to the first andsecond exemplary embodiments.

The above-described exemplary embodiments determine a clear ink amountfor each pixel so as to reduce coloring of an image as a whole. As afifth exemplary embodiment of the present invention, a description willbe given of a method dividing an image and determining a clear inkamount for each pixel so as to reduce coloring of each divided region.In other words, similarly to the fourth exemplary embodiment, assumingthat adjacent pixels have similar pixel values, the fifth exemplaryembodiment aims at a reduction in coloring of an image as a whole byreducing coloring of each divided region.

First, an overview of the fifth exemplary embodiment will be provided.The fifth exemplary embodiment measures coloring of specular reflectionlight when the clear ink amount is changed for each ROB value, similarlyto the first to fourth exemplary embodiments, as an advance preparation.For example, FIG. 18 is a graph schematically illustrating coloring ofspecular reflection light when nine different clear ink amounts arelaid, each of which is plotted on the a*b* plane in the L*a*b colorsystem. The fifth exemplary embodiment selects two colors causingcoloring in a complementary relationship to each other (a relationshipdiagonally located on the a*b* plane), from the measurement results.These selected clear ink amounts A and B are set as a pair. For example,in the example illustrated in FIG. 18, A is 64, and B is 192.

FIG. 19 schematically illustrates the principle of the fifth exemplaryembodiment. In FIG. 19, a pre-division image 1901 contains pixels 1902.The dotted lines indicate boundaries between pixels, and the respectivepixels have respective RGB values. In a post-division image 1903, theimage is divided into square regions, each of which contains four pixelsconsisting of vertically adjacent two pixels and horizontally adjacenttwo pixels. The solid lines indicate boundaries between the dividedregions. Then, the above-described pair of two clear ink amounts isassigned to an average value of RGB values of pixels existing in adivided region. For example, the clear ink amount A is assigned to theupper left pixel and the lower right pixel in a divided region and theclear ink amount B is assigned to the upper right pixel and the lowerleft pixel in the divided region, so that the clear ink amount A and theclear ink amount B are alternately located. Determining a clear inkamount in this way results in establishment of a complementaryrelationship between coloring phenomena at respective pixels, wherebythe coloring phenomena can be canceled out each other in a dividedregion, leading to a reduction in global coloring.

Next, the fifth exemplary embodiment will be described in more detail.This description will be briefly given, mainly focusing on differencesfrom the above-described exemplary embodiments. FIG. 20 is a blockdiagram illustrating a functional configuration of the clear ink amountdetermination processing according to the fifth exemplary embodiment. Animage division unit 2001 divides an image expressed by 8-bit RGB imagedata input from the color matching 502, into square regions eachcontaining four pixels consisting of vertically adjacent two pixels andhorizontally adjacent two pixels, as illustrated in FIG. 19. Then, theimage division unit 2001 outputs the divided image data to the clear inkamount determination processing 504. The clear ink amount determinationprocessing 504 determines a clear ink amount for each of the pixels in aregion of interest divided by the image division unit 2001. The clearink amount determination processing 504 determines clear ink amounts forall of the divided regions. Further, as described above, the clear inkamount determination processing 504 determines a clear ink amount byreferring to a pre-formed specular reflection light coloring table 2003.Therefore, the method of forming the specular reflection light coloringtable 2003 will be now described.

FIG. 21 illustrates an example of the specular reflection light coloringtable 2003. The table 2003 stores RGB values, which are input signalvalues, in the form of 8-bit data in the first to third columns. Thetable 2003 further stores two clear ink amounts causing coloringphenomena in a complementary relationship to each other for each RGBvalue in the fourth and fifth columns. The specular reflection lightcoloring table 2003 is formed based on data acquired from the processingillustrated in the flowchart of FIG. 7. More specifically, first, twokinds of coloring are selected from eight kinds of coloring of specularreflection light acquired from the processing illustrated in theflowchart of FIG. 7, i.e., (ai*, bi*) (i=1 to 8). There are 8C2=28choices in this case. In case that the selected two kinds of coloringare expressed as (a1*, b1*) and (a2*, b2*), the average value ofsaturations of the two kinds of coloring is expressed by the followingequation.

√{square root over ((a1*+a2*)/2)²+((b1*+b2*)/2)²)}{square root over((a1*+a2*)/2)²+((b1*+b2*)/2)²)}  [Math.4]

This average value is also calculated for all of the other combinations,and then the combination having the smallest average value among thecalculated average values is identified. The clear ink amounts of therespective items in the identified combination are stored in the fourthcolumn and the fifth column of the specular reflection light coloringtable 2003. The specular reflection light coloring table 2003 is formedby performing this calculation for all RGB values.

Next, an operation of the clear ink amount determination processing 504will be described with reference to the flowchart illustrated in FIG.22.

First, in step S2201, the clear ink amount determination processing 504inputs divided image data of a region of interest divided by the imagedivision unit 2001 and constituted by four pixels. In step S2202, theclear ink amount determination processing 504 calculates an averagevalue of the respective RGB values of the input four pixels. In stepS2203, the clear ink amount determination processing 504 refers to thespecular reflection light coloring table 2003, and acquires two clearink amounts (CL1 and CL2) corresponding to the RGB value calculated instep S2202. In step S2204, the clear ink amount determination processing504 determines clear ink amounts in such a manner that the CL1 is set asthe clear ink amounts on the upper left and lower right pixels of thedivided image and the CL2 is set as the clear ink amounts on the upperright and lower left pixels of the divided image. In step S2205, theclear ink amount determination processing 504 determines whether stepsS2201 to S2204 are performed to all of the pieces of divided image data.In case that the determination result is NO (NO in step S2205), theprocessing returns to step S2201. On the other hand, in case that thedetermination result is YES (YES in step S2205), the processing isended.

The image division method is not limited to the method of dividing animage into square regions each constituted by four pixels of verticallyadjacent two pixels and horizontally adjacent two pixels. An image maybe divided into any ranges making coloring visually inconspicuous suchas square regions each constituted by nine pixels of vertically adjacentthree pixels and horizontally adjacent three pixels, or rectangularregions each constituted by vertically adjacent two pixels andhorizontally adjacent three pixels. In other words, an image may bedivided by any division method. Further, an image may be divided intoregions having various sizes.

Further, the assignment of clear ink amounts in a divided image is notlimited to the above-described method. The CL1 may be assigned to upperleft and lower left pixels, and the CL2 may be assigned to upper rightand lower right pixels. Alternatively, the CL1 and CL2 may be assignedto not only two pixels but also another number of pixels, respectively.The number of pixels to which the CL1 is assigned may be any of zero tofour depending on the clear ink amount.

Further, three or more kinds of clear ink amounts may be adopted,instead of two kinds of clear ink amounts. For example, as illustratedin FIG. 23, the points of coloring with the clear ink amounts 32, 96,and 192 are set as vertices on the a*b* plane. The three kinds ofcoloring with three clear ink amounts can be positioned inside thetriangle defined by the vertices by weighting the coloring of thevertices. The coloring can be reduced by selecting three clear inkamounts so that the origin point on the a*b* plane is located insidethis triangle, and calculating the ratio of weighting to the respectiveselected ink amounts based on the distances from the origin point, i.e.,the strengths of coloring. In this case, the specular reflection lightcoloring table 2003 stores the ratio, as illustrated in FIG. 24. Then,the clear ink amount determination processing 2002 determines a clearink amount to reduce coloring of specular reflection light per pixel ina divided image so as to satisfy this ratio.

According to this processing, a clear ink amount is determined for eachdivided image to thereby reduce global coloring of specular reflectionlight.

The above-described exemplary embodiments determine a clear ink amountfor a whole image. However, an image before a clear ink is overlaidthereon may contain image regions having different degrees ofconspicuousness of specular reflection light coloring, and it may beredundant to overlay a clear ink on a region with inconspicuous coloringfor the purpose of reducing the coloring. Therefore, as a sixthexemplary embodiment of the present invention, a description will begiven of a method of locating an image region having conspicuouscoloring, and overlaying a clear ink on the located image region toreduce the coloring. In the following, a method of analyzing a frequencyof an image and a method of utilizing a specular reflection lightcoloring table will be described as the method of locating an imageregion having conspicuous coloring of specular reflection light. Thedescription will be briefly given, mainly focusing on differences fromthe above-described exemplary embodiments.

Coloring of specular reflection light is characterized in that itbecomes conspicuous on a solid flat image region in an image. A non-flatimage region can reduce global coloring, because various kinds ofcoloring occur since the RGB value is largely changed on the surface ofsuch an image, resulting in cancelling out the coloring. On the otherhand, a flat image region cannot easily reduce coloring, becausegenerated coloring tends to be in a specific color phase since the RGBvalue is slightly changed on the surface of such an image. Therefore, amethod of detecting a pixel in a flat image region in an image will benow described.

FIG. 25 is a block diagram illustrating a functional configuration ofthe clear ink determination processing with use of a frequency analysisaccording to the present exemplary embodiment. An input unit 2502 inputsRGB image data from the application 501. A frequency analysis unit 2501performs pre-processing for the clear ink amount determinationprocessing 504. The frequency analysis unit 2501 analyzes the frequencyof the RGB image data input from the input unit 2502, and divides thedata into pixels of high-frequency domain and pixels of low-frequencydomain. The frequency analysis unit 2501 outputs pixels of low-frequencydomain to the clear ink amount determination processing 504, and outputspixels other than pixels of low-frequency domain to the gamma correctionprocessing 505. A fixed value (for example, zero) is set as a clear inkamount for pixels other than pixels of low-frequency domain. Further,the frequency analysis processing may be performed by a known method.

According to the above-described processing, a flat region in an imageis located, and a clear ink amount is determined for only that region.As a result, it becomes possible to reduce coloring by overlaying aclear ink only on the region having conspicuous coloring of specularreflection light.

FIG. 26 is a block diagram illustrating a functional configuration ofthe clear ink amount determination processing with use of the specularreflection light coloring table according to the present exemplaryembodiment. A processing pixel determination unit 2601 performspre-processing for the clear ink amount determination processing 504. Aretention unit 2602 retains a predetermined threshold value (forexample, C*=10) defining an allowable range of coloring of specularreflection light.

The processing pixel determination unit 2601 refers to the specularreflection light coloring table to acquire coloring when a clear ink isnot overlaid thereon (when the clear ink amount is zero), for each RGBvalue of input ROB image data. The processing pixel determination unit2601 compares this coloring and the coloring threshold value retained bythe retention unit 2602. As a result of the comparison, in case that theacquired coloring is greater than the coloring threshold value, theprocessing pixel determination unit 2601 outputs the ROB valuecorresponding to the acquired coloring to the clear ink amountdetermination processing 504. In case that the acquired coloring issmaller than the coloring threshold value, the processing pixeldetermination unit 2601 outputs it to the gamma correction unit 505.

According to the above-described processing, a region having conspicuouscoloring is located, and a clear ink amount is determined for only thelocated region. As a result, it becomes possible to reduce coloring byoverlaying a clear ink only on the region having conspicuous coloring ofspecular reflection light.

Further, the above-described exemplary embodiments have been describedbased on an example of overlaying a clear ink, but the present inventionis not limited thereto. That is, the discharge data of a clear ink,which is processed by the mask processing of the mask data conversionunit, may not be laid on the outermost surface of a sheet.

Further, the present invention can be also embodied by providing asystem or an apparatus with a storage medium storing program codes ofsoftware capable of realizing the functions (for example, the functionsindicated in the above-described flowcharts) of the above-describedexemplary embodiments. In this case, a computer (or a CPU or a microprocessing unit (MPU)) of the system or the apparatus reads out andexecutes the program codes that is stored in the storage medium in acomputer readable manner, thereby realizing the functions of theabove-described exemplary embodiments.

Further, the above-described exemplary embodiments may be used as acombination of any of them. According to the present invention, it ispossible to provide image processing capable of comprehensively reducingcoloring of specular reflection light.

Aspects of the present invention can also be realized by a computer of asystem or apparatus (or devices such as a CPU or MPU) that reads out andexecutes a program recorded on a memory device to perform the functionsof the above-described embodiment(s) and by a method, the steps of whichare performed by a computer of a system or apparatus by, for example,reading out and executing a program recorded on a memory device toperform the functions of the above-described embodiment(s). For thispurpose, the program is provided to the computer for example via anetwork or from a recording medium of various types serving as thememory device (e.g., computer-readable medium).

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all modifications, equivalent structures, and functions.

This application claims priority from Japanese Patent Application No.2010-169602 filed Jul. 28, 2010, which is hereby incorporated byreference herein in its entirety.

1. An image processing apparatus configured to convert image data intocolor material data including color material data indicating a colormaterial amount of a chromatic color material and color material dataindicating a color material amount of an achromatic color material, theimage processing apparatus comprising: a conversion unit configured toconvert image data of a region of interest in an image into the colormaterial data of the chromatic color material; and a determination unitconfigured to determine the color material data of the achromatic colormaterial corresponding to the image data so that a color indicated bycoloring information corresponding to the image data of the region ofinterest in the image approaches an achromatic color.
 2. The imageprocessing apparatus according to claim 1, wherein the determinationunit determines, as the color material data of the achromatic colormaterial corresponding to the image data, color material data of theachromatic color material corresponding to coloring information suchthat a color indicated by the coloring information corresponding to theimage data of the region of interest is closest to the achromatic coloramong coloring information corresponding to a combination of theconverted color material data of the chromatic color material and colormaterial data of the achromatic color material.
 3. The image processingapparatus according to claim 1, wherein the region of interest is aregion comprising a plurality of pixels, and wherein the determinationunit determines the color material data of the achromatic color materialon the region of interest by using a combination such that coloringinformation pieces corresponding to respective image data pieces of theplurality of pixels cancel out each other.
 4. The image processingapparatus according to claim 3, wherein the determination unitdetermines the color material data of the achromatic color material onthe region of interest by respectively assigning the coloringinformation pieces corresponding to the respective image data pieces ofthe plurality of pixels to alternately located pixels in the region ofinterest.
 5. The image processing apparatus according to claim 4,wherein the coloring information pieces constituting the combinationthat cancel out each other are two coloring information pieces.
 6. Theimage processing apparatus according to claim 5, wherein thedetermination unit assigns the two coloring information pieces afterweighting them according to a strength of coloring thereof.
 7. The imageprocessing apparatus according to claim 1, further comprising arecording unit configured to record the achromatic color material on arecording medium based on the determined color material data of theachromatic color material. wherein the recording unit records theachromatic color material after recording the chromatic color material.8. The image processing apparatus according to claim 1, furthercomprising a resolution conversion unit configured to convert aresolution of the image data. wherein the determination unit determinesthe color material data of the achromatic color material on the regionof interest of the image data with the converted resolution.
 9. Theimage processing apparatus according to claim 1, further comprising adetermining unit configured to determine an image region of the imagedata, wherein the determination unit determines the color material dataof the achromatic color material on the region of interest in thedetermined region.
 10. The image processing apparatus according to claim9, wherein the determining unit performs a frequency analysis on theimage data and, as a result of the frequency analysis, determines apixel of low-frequency domain.
 11. The image processing apparatusaccording to claim 9, wherein the determining unit determines the imageregion based on the coloring information corresponding to thecombination of the converted color material data of the chromatic colormaterial and the determined color material data of the achromatic colormaterial and a threshold value.
 12. The image processing apparatusaccording to claim 1, wherein the coloring information is a measurementvalue acquired by measuring a patch image generated with use of thecolor material data.
 13. The image processing apparatus according toclaim 1, wherein the achromatic color material is a clear ink.
 14. Animage processing method for an configured to convert image data intocolor material data including color material data indicating a colormaterial amount of a chromatic color material and color material dataindicating a color material amount of an achromatic color material, theimage processing method comprising: converting image data of a region ofinterest in an image into the color material data of the chromatic colormaterial; and determining the color material data of the achromaticcolor material corresponding to the image data so that a color indicatedby coloring information corresponding to the image data of the region ofinterest in the image approaches an achromatic color.
 15. Anon-transitory computer-readable storage medium storing a programcausing an image processing apparatus to perform the image processingmethod according to claim 14.